Naturally occurring water supplies, including ground water, often contain various minerals in different degrees, including such metal ions as iron, manganese, aluminum, chromate, calcium, magnesium, zinc, nickel, and the like. Generally, it is customary to filter these water supplies, particularly if they contain solids, by filtration, which, however does not remove any of the dissolved metal salts.
Accordingly, when water is treated to remove the volatile organic contaminants (VOC's) in a packed column, such as an air stripper, the metal ions, particularly iron and magnanese, precipitate as oxides, resulting in a fouled system. Ground water is also often contaminated with bacteria which dramatically increase the volume and rate of fouling of the air stripper and reinjection system. More specifically, a packed column used as an air stripper is a common approach to removing volatile organic contaminants (VOC's) from ground water. The air strippers function by passing the contaminated water through a tower, through which a large volume of air is blown in a direction counter to the flow of the water. The VOC's pass from the water into the air, leaving a cleaner water. The water is then put through a secondary treatment, or pumped directly back into the ground. Fouling of the reinjection basins or wells by iron oxides, manganese oxides, and bacteria is often a serious problem.
Approximately half the population of the United States uses ground water for drinking and various other purposes, and many agricultural industries depend on clean and safe ground water. Generally, the water flows straight from the ground into use with little treatment. Most of the consumption is from privately-owned wells, and there is comparatively little treatment before the water goes into use. However, even though packed column air strippers are found to be the most economical approach to cleaning ground water, they are often plagued with operating problems. Specifically, unless there is proper treatment to control mineral deposits and biological growth, fouling will build up on the tower surfaces and in the reinjection wells, thereby reducing efficiency. The contamination of the underground water supplies, however, comes from various hazardous materials, including, for example, garbage dumps, industrial waste, septic tanks, leaking storage tanks, e.g., gasoline tanks, and the like. With the growing problem of contaminated underground water, much is being done to minimize the potential for contamination through prevention and remediation.
Contamination of ground water caused by volatile organic compounds is of major concern, since most of the volatile contaminants enter the ground water from industrial spills, leaking pipelines and storage tanks. Accordingly, some of the most common forms of volatile contaminants are gasoline and related organic materials escaping from underground tanks and traveling readily with underground water flow. If the water were left untreated, the contamination would produce acute toxic effects as well as serious health problems. Remediation of volatile contaminants in ground water at a particular site begins with an assessment of the chemistry involved and the nature of the local aquifer. The volatilization of organic material from contaminated ground water is generally accomplished by the use of an air stripper. For example, an aerator blows air through the water to remove the volatile materials, but then may require some other method for removing the non-volatile materials.
In theory, all organic contaminations are treatable by a biological method. Since organics are composed mostly of carbon, hydrogen, and oxygen, they are a good source of food for microorganisms. Most of the biological treatments bring the contaminated water to the surface and run it through a biological reactor. The biological cleanup programs require a sensitive balance of the organic contaminants with the metabolic needs of the microorganisms. Cleanup programs can be enhanced by using air strippers to remove the bulk of volatile contaminants before passing water into a bioreactor to remove the non-volatile organic materials. The treatment of ground water with an air stripper is simply the use of a tower containing a packing medium through which the water and air flow in countercurrent direction. The relative concentrations of the volatile contaminants in the water volatilize into the air passing through the medium. The volatile contaminants are removed by the packed column stripper and are carried out into the air. Effective air stripping requires that contact between the air and water be maximized. Therefore, the design of the tower or packing in the packed column is critical, and may include several redistribution rings to ensure that the water does not channel along the walls of the stripper. Once a remediation program is established, continuous operation is an essential factor, since any time the air stripper is shut down, the underground water flow pattern changes and control of the contaminants in the ground is reduced. Accordingly, any factor which might take the air stripper off-line must be avoided. One of the most common problems facing air strippers is deposits caused by the natural minerals contained in the water and the biological foulings of the air stripper surfaces. Ground waters that are high in iron and other metal ions are particularly prone to deposition. Virtually all waters are subject to biological foulings.
Iron and various other metal deposits can be controlled through the use of inhibitors at low dosage levels. The chemistry of these inhibitors, however, must be carefully designed to operate at dosages that are sufficiently low to be appropriate for water reinjection. Where the air stripper is the only remediation equipment involved, the use of a microbiocide will eliminate microbiological growth and slime buildup within the air stripper and reinjection system, and therefore minimize biological fouling.